1. Field of the Invention
This invention relates to an apparatus and method for cooling Instrumentation in an apparatus exposed to high temperature environments. In particular, this invention relates to active cooling of instrumentation, such as electronics in a downhole tool positioned in a wellbore.
2. Background Art
The environment encountered by downhole oil exploration tools can be very severe. Temperatures up to and in excess of 200 degree C. and pressures up to 1.38xc3x97108 Pa are not uncommon. Consequently, producers of oil exploration tools must design robust tools that can operationally sustain these harsh conditions for extended lengths of time. Perhaps the most challenging of all conditions to design electronics that can reliably operate in high temperature environments. Standard electronic components are usually rated to operate only up to approximately 125 degree C. Thus, it becomes necessary to create or experimentally find electric components that can survive the high temperatures existing downhole. Since the components are constantly changing via new manufacturing techniques, updates, etc., this process of creating electronic components is expensive, time consuming, and never ending. In an effort to combat the high temperature requirement of electronics, the chassis or electronics compartments in downhole tools could be kept at or below 125 degree C.
Today, tools rated to 175 degree C. are sometimes inserted into Dewar Flasks when exploring boreholes in excess of 175 degree C. Dewar Flasks act to insulate the tool electronics and to slow the heating of the electronic chassis similar to a large xe2x80x9cthermal bottlexe2x80x9d. The flask is a passive system that extends the downhole residence time of the tools by approximately four to six hours. Often the downhole residence times required for exploration are much greater than those offered by the expensive Dewar Flask system.
The problem at hand points toward the need for an active cooling system that can maintain the electronic chassis below 125 degree C. for extended lengths of time. Standard electronics could then be used without the need for the expensive high temperature components.
Active cooling systems already exist for a variety of applications such cooling food products, motor vehicles and buildings. These active cooling systems, better known as air conditioners and refrigerators, can effectively operate for extended periods of time with little to no maintenance. A cooling system makes heat move. It takes heat from one location and moves it to another location. The location from which heat was removed obviously becomes colder. For example, a refrigerator takes heat out of the inside and moves it to the outside. The heat flows into the air and the inside, having lost heat, becomes colder.
Vapor compression active cooling systems work by evaporation. When a liquid turns into a vapor, it loses heat and becomes cooler. This change is because the molecules of vapor need energy to move and leave the liquid. This energy comes from the liquid; the molecules left behind have less energy and so as a result, the liquid is cooler.
For an active cooling system to work continuously, the same cooling agent (etc., Freon) must be repeatedly used for an indefinite period. These cooling systems have three basic patterns: the vapor-compression system, the gas-expansion system and the absorption system. The vapor-compression system is typically more effective and is used more extensively than the other arrangements. The vapor-compression system consists of four main elements: an evaporator, a compressor, a condenser and an expansion device.
Referring to FIG. 1, in the evaporator 1, the cooling agent boils (evaporates) at a temperature sufficiently low to absorb heat from a space or medium that is being cooled. The boiling temperature is controlled by the pressure maintained in the evaporator, since the higher the pressure the higher the boiling point. The compressor 2 removes the vapor as it is formed, at a rate sufficiently rapid to maintain the desired pressure. This vapor is then compressed and delivered to a condenser 3. The condenser dissipates heat to circulating water or air. The condensed liquid cooling agent, which is now ready for use in the evaporator 1, is then sharply reduced in pressure by passing it through an expansion valve 4. Here, the pressure and temperature of the cooling agent drop until they reach the evaporator pressure and temperature, thus allowing the cooling cycle to repeat.
During expansion some of the liquid of the cooling agent flashes into vapor so that a mixture of liquid and flash vapor enters the evaporator. In a cooling system, the low pressure in the evaporator is set by the cooling temperature which is to be maintained. The high pressure maintained in the condenser is determined ultimately by the available cooling medium (e.g., the temperature of circulating water or the atmosphere air temperature). The process is one in which the cooling agent absorbs heat at a low temperature and then under the action of mechanical work, the cooling agent is compressed and raised to a sufficiently high temperature to permit the rejection of this heat. Mechanical work or energy supplied to the compressor as power is always required to raise the temperature of the system.
To further explain the cooling process, the four major components are examined in greater detail The evaporator 1 is the part of the cooling system in which the cooling is actually produced. The liquid cooling agent and vapor from the expansion valve 4 are introduced into the evaporator. As the liquid vaporizes, it absorbs heat at low temperature and cools its surroundings or the medium in contact with it. Evaporators may be direct expansion (acting directly to cool a space or product) or they may operate as indirect-expansion units to cool a secondary medium, such as water or a brine which in turn is pumped to a more distant point of utilization. A domestic refrigerator, for example, is a direct-expansion unit in that its evaporator directly cools the air in the food compartment and also directly contacts the water trays used for making ice. Evaporators vary greatly in design, with those used for cooling air often made as continuous pipe coils, with fins mounted outside the pipes to give greater surface contact to the air being chilled. For cooling liquid, such as a brine water, the shell and tube arrangement is common in this case, the brine passes through tubes surrounded by the boiling (evaporating) cooling agent, which is contained in a larger cylindrical shell. The brine tubes, in turn, are welded or rolled into tube sheets at the end of the shell to prevent leakage of the cooling agent from the shell or into the brine circuit.
The expansion valve 4 that feeds the evaporator must control the flow so that sufficient cooling agent flows into the evaporator for the cooling load but not in such excess that liquid passes over to the compressor, with the possibility of causing damage to it.
The compressor 2, the key element of the system, can be powered by means such as electric motor, steam or internal combustion engine, or steam or gas turbine. Most compressors are of the reciprocating (piston) type and range from the fractional-horsepower size, such as those found in domestic refrigerators or in small air-conditioning units, to the large multi-cylinder units that serve large industrial systems. In these large multi-cylinder units, capacity can be controlled with automatic devices that prevent the in certain cylinders from closing. For example, in a six-cylinder unit, if the valves are held open on two of the cylinders to keep them inoperative, the capacity of the machine is reduced by one-third when operating at normal speed.
Centrifugal compressors are used for large refrigeration units. These compressors employ centrifugal impellers that rotate at high speed. Centrifugal compressors depend for their compression largely on the dynamic action of the gases themselves as they flow in the diffusion passages of the compressor. These compressors can be large centrifugal compressors made with a single impeller or with two to four or more impellers in series, to compress the gas through the range required. These compressors are used extensively for large air-conditioning installations and also for usage in the industrial field when gases are compressed for liquefaction or for transportation, such as in the natural-gas industry, and when air is compressed to produce liquid oxygen or nitrogen.
The condenser 3 of a vapor system must dissipate heat from the hot vapor it receives from the compressor and condense this vapor to liquid for reuse by the evaporator. Condensers either dissipate heat to the ambient atmosphere through externally finned surfaces or by a shell and tube arrangement in which the vapor delivers heat to a circulating fluid (etc.: water) that passes through tubes contacting the cooling agent vapor. The temperature of the vapor is kept above that of the circulating water or air by compression to insure that heat is transferred to the coolant; thus, when the vapor is allowed to expand, its temperature drops well below that of the cooling agent.
Double-pipe condensers are also used. In such units, the cooling agent vapor and condensate pass in one direction through the annular space between the two tubes, while the water, flowing in the opposite direction through the central tube, performs the cooling function.
The air conditioning concept works on the principle of exchanging heat from a heated substance to a cold substance. In this principle, the temperature from a hot substance (such as a fluid) is transferred to a cold fluid. As the temperature of the hot fluid decreases, the temperature of the cold fluid increases. Heat exchangers are manufactured in many different designs and are used extensively in various industries. Heat exchangers are given different names when they serve a special purpose. Thus boilers, evaporators, superheaters, condensers and coolers may all be considered heat exchangers.
An example of a heat exchanger is illustrated in FIG. 2a and explains the basic operation of a heat exchanger. This exchanger is constructed from two pipes 7 and 8 in a concentric arrangement. Inlet and exit pipes 5, 6 are provided for the two fluids. In the sketch, the cold fluid flows through the inner tube 7 and the warm fluid via inlet pipe 5 in the same direction through the annular space between the outer and the inner tube. This flow arrangement is called parallel flow. In it heat is transferred from the warm fluid through the wall of the inner tube (the so-called heating surface) to the cold fluid. The temperature in both fluids varies as shown in FIG. 2b. In plot 9, the temperature of the warm fluid decreases from txcfx891to txcfx892. In plot 10, the temperature of the cold fluid increases from tc1 to tc2.The amount of heat Q that is transferred from one fluid to the other per unit of time, called heat flow, can be calculated from the following equation: Q=m c (t2t1)(1)This equation states that the heat flow Q(kW) can be obtained by multiplying the mass per unit of time of fluid m(kg/sec) by the specific heat c(KJ/kg-degree C.) of the fluid and by the temperature increase t1xe2x88x92t2 (degree C.) of the fluid entrance to the exit of the heat exchanger. The specific heat is a property of the fluid involved and its current state. The amount of heat leaving the warm fluid must be the same as the amount of heat received by the cold fluid. The mass flow and the temperature increase for the cold or the decrease for the warm fluid can therefore be entered into equation (1). The heat exchanger may have to be designed, for example, to increase the temperature of a prescribed mass per unit time m2 of cold fluid from tc1 to tc2. Entering these value into equation (1) then determines the heat flux Q which has to be transferred in the heat exchanger. This value will be needed in the following discussion to calculate the heating surface of the exchanger.
The temperature difference xcex94t1 between the fluids at the entrance of the heat exchanger decreases to the value xcex94t2 at the exit, as illustrated in FIG. 2a. A heat exchanger is operated in counterflow when the direction of one of the fluids is reversed. The counterflow arrangement has the advantage that the exit temperature tc1 of the colder fluid can be increased beyond the exit temperature txcfx892 of the warm fluid. In addition, a smaller surface area is required in counterflow than in parallel flow to transfer the same amount of heat. This is so because the mean temperature difference xcex94tm in the counterflow heat exchanger, for a given heat flux and prescribed inlet temperatures, is higher than in the parallel-flow exchanger.
The heating surface of the heat exchanger can be obtained from the equation:                     A        =                  Q                      U            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢                          l              m                                                          (        2        )            
The equation indicates that the required surface area A (m2) is obtained by dividing the heat flux Q obtained with equation (1) by the overall heat transfer coefficient U and the mean temperature difference xcex94tm (degree C.). Larger heat exchangers utilize a bundle of tubes through which one of the fluids flows. The tubes are enclosed in a shell with provisions for the other fluid to flow through the spaces between the tubes. Fluid flowing outside the tubes can be directed either in the same direction as or counter to the effective flow in the tube bundles. In the latter arrangement, parallel or counter flow can be approximated in the way shown in FIG. 2a. In another arrangement, the cold fluid is distributed in such a manner that it flows in parallel through the tubes forming the heating surface and is then collected by a header. This arrangement creates a cross flow, as shown schematically in FIG. 2a. In nuclear reactors, fuel rods may replace the tubes, and the cooling fluid flowing around the rods removes the heat generated by the fission process. In a similar way, rods containing electric resistance heaters may supply heat to the fluid passing through the exchanger between the rods.
As previously mentioned, there is a need for a downhole cooling system that can keep downhole tool electronics cool in order to avoid tool failure from the extreme downhole temperatures. There have been attempts to apply the refrigeration concept to downhole tools. In 1977, Mechanics Research attempted to develop a system that incorporated a refrigeration technique for use in a geothermal well. The system design was to be a closed system that would operate continuously, similar to the refrigerator cooling concept of FIG. 1. However, the specific objective of the project was to develop a compressor for such a system. The project did not achieve its chief objectives.
Other techniques have also been developed to provide cooling for electronics. For example, U.S. Pat. No. 5,701,751 to Flores, assigned to the assignee of the present invention, provides a system for actively cooling instrumentation in a high temperature environment. This patent uses a hot heat exchanger 13, cold heat exchanger 12 and a compressor 11 to pump fluids through a downhole tool to cool the instrumentation. It is, however, limited in its maximum operating time, since it is based on a once through cycle, without the capability to re-circulate the fluid. As electronics are now used for extended durations in the drilling environment and in other newly instrumented downhole operations, the performance needs of the cooling system must increase. It is, therefore, desirable to provide a system capable of one or more of the following advantages (among others): continuous operation, reduced or eliminated time constraints, cooling of electronics to lower regulating temperatures, active cooling of electronics, insulation of electronics, added protection from outside elements, cooling over extended periods of time, layered protection and/or cooling of instrumentation, and cooling systems compatible with high temperature wellbore operations.
An active cooling system for downhole operations is provided. In one aspect, the present invention relates to an apparatus for actively cooling instrumentation contained in a downhole tool. The apparatus comprises a compressor, a condenser, an expansion valve and a heat exchanger (or evaporator). The compressor pressurizes a cooling fluid. The condenser is in fluid communication with the compressor and is capable of converting the cooling fluid from vapor into liquid. The expansion valve is in fluid communication with the condenser and is capable of selectively releasing the cooling fluid. The heat exchanger is adapted to receive the cooling fluid from the expansion valve and return the cooling fluid to the compressor. The heat exchanger comprises an inner container positioned about the instrumentation, and an outer container positioned about the inner container. The inner container is in fluid communication with the expansion valve, and the outer container is in fluid communication with the inner container and the compressor. The cooling fluid is cooled as it flows through the compressor, condenser and expansion valve, and absorbs heat as it passes through the inner and outer containers whereby the instrumentation remains cool in another aspect, the invention relates to a method for cooling instrumentation in a downhole tool. The method comprises pressurizing a cooling fluid, condensing the pressurized cooling fluid, passing a cooling fluid through an inner container positioned about the instrumentation, and passing the cooling fluid through an outer container disposed about the inner container.
In yet another aspect, the invention relates to a method for cooling instrumentation in a downhole tool. The method comprises positioning the downhole tool in a wellbore. The downhole tool comprises an inner container positioned about the instrumentation and an outer container positioned about the inner container. Cooling fluid is passed through the inner container, the outer container and into a compressor. The fluid is pressurized and passed into a condenser. The fluid is condensed into a liquid and released back into the inner container.
In yet another aspect, the invention relates to an apparatus for cooling instrumentation in a downhole tool disposable in a wellbore. The apparatus comprises an inner container disposed about the instrumentation, an outer container disposed about the inner container, a compressor, a condenser and an expansion valve. The outer container is in fluid communication with the inner container. The compressor is in fluid communication with the outer container and adapted to pressurize cooling fluid. The condenser is in fluid communication with the compressor. The condenser is adapted to convert the fluid from vapor to liquid. The expansion valve is in fluid communication with the condenser and the inner container. The expansion valve is capable of selectively releasing cooling fluid into the inner container. The cooling fluid flows through the compressor, the condenser and the expansion valve whereby the cooling fluid is cooled and released into the inner container, and the cooling fluid flows through the inner and outer containers whereby the instrumentation is cooled.
Finally, in another aspect, the invention relates to an apparatus for cooling instrumentation in a downhole tool. The apparatus comprises a compressor, a condenser, a valve, and a heat exchanger. The heat exchanger is positioned about the instrumentation and has an inner chamber and an outer chamber. The inner chamber is in fluid communication with the outer chamber. Fluid flows through the inner chamber and the outer chamber and removes heat therefrom as it flows therethrough. The heated fluid passes into the compressor for pressurization therein. The condenser converts the fluid from vapor to liquid, and the valve selectively releases the fluid into the inner chamber upon cooling whereby the instrumentation is cooled.
The system allows constant low pressure vaporization of the cooling fluid as it passes through the heat exchanger. The heat from the electronics, as well as that from the hot borehole (up to approximately 200 C.) causes the water/steam mixture coming out of the expansion valve to boil and vaporize entirely into steam. In doing so, it extracts heat from the electronics payload and from the containing walls as it travels toward the compressor. The path of the vapor is designed so that it provides insulation between the electronics and the outside by providing a double layer moving heat away from the electronics and towards the compressor. As the steam is pulled by the compressor, its pressure and resulting temperature can be regulated thereby regulating the temperature of the electronics. For instance, the temperature of lower tank can be maintained at approximately 100 degrees C. if its internal pressure is kept at approximately 1.01xc3x97105 Pa (14.7 psi). The vapor is typically compressed to a pressure greater than the saturation pressure of the steam at the temperature of the borehole. A 200 degrees C. borehole would require a pressure of 1.55xc3x97106 Pa (225 psi). A control system may be provided to maintain a constant pressure vaporization.
As the instruments and/or the wellbore operation generate heat, the heat is transferred through the heat exchanger to the cooling fluid. The cooling fluid boils and vaporizes from the heat and is pumped out of the heat exchanger by a compressor. As the cooling fluid vaporizes and is pumped out of the heat exchanger, heat contained in the cooling fluid is transferred out as well. The rate at which the vapor is pumped out controls the temperature of the cooling fluid in the heat exchanger. The vapor is compressed under pressure and pumped to a condenser where the vapor condenses back to a liquid.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.